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Fundamentals of Radiation |
- Basic Terms
- The Atom
- Radioactive Decay
- Alpha Particle Radiation
- Beta Particle Radiation
- Gamma Ray Radiation
- X-Ray Radiation
- Units of Radiation Measurement
- Half-Life
Radiation
Radiation is energy in transit in the form of high speed particles and electromagnetic waves. We encounter electromagnetic waves every day. They make up our visible light, radio and television waves, ultra violet (UV), and microwaves with a large spectrum of energies. These examples of electromagnetic waves do not cause ionizations of atoms because they do not carry enough energy to separate molecules or remove electrons from atoms.Ionizing radiation
Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from their orbits, causing the atom to become charged or ionized. Examples are gamma rays and neutrons.Non-ionizing radiation
Non-ionizing radiation is radiation without enough energy to remove tightly bound electrons from their orbits around atoms. Examples are microwaves and visible light.Radioactivity
Radioactivity is the spontaneous transformation of an unstable atom and often results in the emission of radiation. This process is referred to as a transformation, a decay or a disintegration of an atom.Radioactive Material
Radioactive Material is any material that contains radioactive atoms.Radioactive Contamination
Radioactive contamination is radioactive material distributed over some area, equipment or person. It tends to be unwanted in the location where it is, and has to be cleaned up or decontaminated.Electron Volt
Ionizing radiation is measured in a basic unit called an electron volt. This unit of energy is the kinetic energy of an electron accelerated through a potential of 1 volt. Larger multiples of the electron volt are expressed as KeV for thousands of electron volts and MeV for millions of electron volts.
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
The Bohr Model of the atom consists of a central nucleus composed of neutrons and protons surrounded by a number of orbital electrons equal to the number of protons.
Protons are positively charged, while neutrons have no charge. Each has a mass of about 1 atomic mass unit or amu. Electrons are negatively charged and have mass of 0.00055 amu.
The number of protons in a nucleus determines the element of the atom. For example, the number of protons in uranium is 92 while the number in neon is 10. The proton number is often referred to as Z.
An element may have several isotopes. An isotope of an element is comprised of atoms containing the same number of protons as all other isotopes of that element, but each isotope has a different number of neutrons than other isotopes of that element. Isotopes may be expressed using the nomenclature Neon-20 or 20Ne10, where 20 represents the combined number of neutrons and protons in the atom (often referred to as the mass number A), and 10 represents the number of protons (the atomic number Z).
While many isotopes are stable, others are not. Unstable isotopes normally release energy by undergoing nuclear transformations (also called decay) through one of several radioactive processes described later in this module.
Elements are arranged in the periodic table with increasing Z. Radioisotopes are arranged by A and Z in the chart of the nuclides.
Go to a detailed periodic table of the nuclides.
Radiation is energy traveling in the form of particles or waves in bundles of energy called photons. Some everyday examples are microwaves used to cook food, radio waves for radio and television, light, and x-rays used in medicine.
Radioactivity is a natural and spontaneous process by which the unstable atoms of an element emit or radiate excess energy in the form of particles or waves. These emissions are collectively called ionizing radiations. Depending on how the nucleus loses this excess energy either a lower energy atom of the same form will result, or a completely different nucleus and atom can be formed.
Ionization is a particular characteristic of the radiation produced when radioactive elements decay. These radiations are of such high energy that when they interact with materials, they can remove electrons from the atoms in the material. This effect is the reason why ionizing radiation is hazardous to health, and provides the means by which radiation can be detected.
Examples of ionizing radiation include:
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Non ionizing radiations are not energetic enough to ionize atoms and interact with materials in ways that create different hazards than ionizing radiation. Examples of non ionizing radiation include:
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The atomic structure for certain isotopes of elements is naturally unstable. Radioactivity is the natural and spontaneous process by which the unstable atoms of an isotope of an element transform or decay to a different state and emit or radiate excess energy in the form of particles or waves. These emissions are energetic enough to ionize atoms and are called ionizing radiation. Depending on how the nucleus loses this excess energy, either a lower energy atom of the same form results or a completely different nucleus and atom is formed.
A given radioactive isotope decays through a specific transformation or set of transformations. The type of emissions, along with the energy of the emissions, that result from the radioactive decay are unique to that isotope. For instance, an atom of Phosphorus-32 decays to an atom of non-radioactive Sulfur-32, accompanied by the emission of a beta particle with an energy up to 1.71 million electron-volts.
The following sections describe the radiations associated with the radioactive decay of the radioisotopes most commonly used in research at The University of Utah.
An alpha particle consists of two neutrons and two protons ejected from the nucleus of an atom. The alpha particle is identical to the nucleus of a Helium atom.
Examples of alpha emitters are Radium, Radon, Thorium, and Uranium.
Because alpha particles are charged and relatively heavy, they interact intensely with atoms in materials they encounter, giving up their energy over a very short range. In air, their travel distances are limited to no more than a few centimeters. As shown in the following illustration, alpha particles are easily shielded against and can be stopped by a single sheet of paper.
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
Since alpha particles cannot penetrate the dead layer of the skin, they do not present a hazard from exposure external to the body.
However, due to the very large number of ionizations they produce in a very short distance, alpha emitters can present a serious hazard when they are in close proximity to cells and tissues such as the lung. Special precautions are taken to ensure that alpha emitters are not inhaled, ingested or injected.
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
A beta particle is an electron emitted from the nucleus of a radioactive atom.
Examples of beta emitters commonly used in biological research are: Hydrogen-3 (tritium), Carbon-14, Phosphorus-32, Phosphorus-33, and Sulfur-35.
Beta particles are much less massive and less charged than alpha particles and interact less intensely with atoms in the materials they pass through, which gives them a longer range than alpha particles. Some energetic beta particles, such as those from P-32, will travel up to several meters in air or tens of mm into the skin, while low energy beta particles, such as those from H-3, are not capable of penetrating the dead layer of the skin. Thin layers of metal or plastic stop beta particles.
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
All beta emitters, depending on the amount present, can pose a hazard if inhaled, ingested or absorbed into the body. In addition, energetic beta emitters are capable of presenting an external radiation hazard, especially to the skin.
An important consideration in shielding beta particle radiation is the ability of beta particles to produce a secondary radiation called Bremsstrahlung. Bremsstrahlung are X-rays produced when beta particles or other electrons decelerate while passing near the nuclei of atoms. The intensity of Bremsstrahlung radiation is proportional to the energy of the beta particles and the atomic number of the material through which the betas are passing.
Consequently, Bremsstrahlung radiation is generally not a concern for lower energy beta emitters such as Carbon-14 and Sulfur-35, but the higher energy betas from Phosphorus-32 can produce significant Bremsstrahlung, especially when passing through shielding materials such as lead. Lower atomic number materials such as Plexiglas are preferred shielding materials for high energy emitters such as Phosphorus-32.
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
A gamma ray is a packet (or photon) of electromagnetic radiation emitted from the nucleus during radioactive decay and occasionally accompanying the emission of an alpha or beta particle. Gamma rays are identical in nature to other electromagnetic radiations such as light or microwaves but are of much higher energy.
Examples of gamma emitters are Cobalt-60, Zinc-65, Cesium-137, and Radium-226.
Like all forms of electromagnetic radiation, gamma rays have no mass or charge and interact less intensively with matter than ionizing particles. Because gamma radiation loses energy slowly, gamma rays are able to travel significant distances. Depending upon their initial energy, gamma rays can travel tens or hundreds of meters in air.
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
Gamma radiation is typically shielded using very dense materials (the denser the material, the more chance that a gamma ray will interact with atoms in the material) such as lead or other dense metals.
Gamma radiation particularly can present a hazard from exposures external to the body.
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
Like a gamma ray, an X-ray is a packet (or photon) of electromagnetic radiation emitted from an atom, except that the X-ray is not emitted from the nucleus. X-rays are produced as the result of changes in the positions of the electrons orbiting the nucleus, as the electrons shift to different energy levels.
Examples of X-ray emitting radioisotopes are Iodine-125 and Iodine-131.
X-rays can be produced during the process of radioactive decay or as Bremsstrahlung radiation. Bremsstrahlung radiation are X-rays produced when high-energy electrons strike a target made of a heavy metal, such as tungsten or copper. As electrons collide with this material, some have their paths deflected by the nucleus of the metal atoms. This deflection results in the production of X-rays as the electrons lose energy. This is the process by which an X-ray machine produces X-rays.
Like gamma rays, X-rays are typically shielded using very dense materials such as lead or other dense metals.
X-rays particularly can present a hazard from exposures external to the body.
Units of Radiation Measurement
(diagram courtesy of the University of Michigan Student Chapter of the Health Physics Society )
Quantity
The quantity of radioactive material present is generally measured in terms of activity rather than mass, where activity is a measurement of the number of radioactive disintegrations or transformations an amount of material undergoes in a given period of time. Activity is related to mass, however, because the greater the mass of radioactive material, the more atoms are present to undergo radioactive decay.
The two most common units of activity are the Curie or the Becquerel (in the SI system).
| 1 Curie (Ci) = 3.7 x1010 disintegrations per second (dps) |
| 1 Becquerel (Bq) = 1 disintegration per second (dps). |
Obviously, 1 Curie is a large amount of activity, while 1 Becquerel is a small amount. In the typical University of Utah laboratory, millicurie and microcurie (or kilo and MegaBecquerel) amounts of radioactive material are used.
| 1 millicurie = 2.2 x 109 disintegrations per minute (dpm) = 3.7 x 107 Bq = 37 MBq |
| 1 microcurie = 2.2 x 106 dpm = 3.7 x 104Bq = 37 kBq |
Intensity
For the purposes of radiation protection, it is not always useful to describe the potential hazard of a radioactive material in terms of its activity. For instance, 1 millicurie of Phosphorus-32 a centimeter from the body poses a much different hazard than 1 millicurie of tritium a centimeter from the body.
Consequently, it is often preferable to measure radiation by describing the effect of that radiation on the materials through which it passes. The three main quantities which describe radiation field intensity are defined as follows:
Common Units - USA
These are the common units used in the United States in health physics.Roentgen (R)
The roentgen is a unit used to measure a quantity called exposure. This can only be used to describe an amount of gamma and X-rays, and only in air. One roentgen is equal to depositing in dry air enough energy to cause 2.58E-4 coulombs per kg. It is a measure of the ionizations of the molecules in a mass of air. The main advantage of this unit is that it is easy to measure directly, but it is limited because it is only for deposition in air, and only for gamma and X rays.Rad (radiation absorbed dose)
The rad is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One rad is defined as the absorption of 100 ergs per gram of material. The unit rad can be used for any type of radiation, but it does not describe the biological effects of the different radiations.Rem (roentgen equivalent man)
The rem is a unit used to derive a quantity called equivalent dose. This relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of thousandths of a rem, or milirem. To determine equivalent dose (rem), you multiply absorbed dose (rad) by a quality factor (Q) that is unique to the type of incident radiation. Radiation dose limits are expressed in units of mrem or rem.SI Prefixes
Many units are broken down into smaller units or expressed as multiples, using standard metric prefixes. As examples, a kilobecquerel (kBq) in 1000 becquerels, a millirad (mrad) is 10-3 rad, a microrem (µrem) is 10-6 rem, a nanogram is 10-9 grams, and a picoCurie is a 10-12 Curies.| Quantity | Unit | What is measured | Amount |
| Exposure | Roentgen (R) Coulombs/kg |
Amount of charge produced in 1 kg of air by x- or gamma rays |
1 R = 2.58 x 10-4 Cb/kg |
| Absorbed Dose | Rad Gray (Gy) |
Amount of energy absorbed in 1 gram of matter from radiation | 1 Rad = 100 ergs*/gram
1 Gy = 100 rad |
| Dose Equivalent | Rem Sievert (Sv) |
Absorbed dose modified by the ability of the radiation to cause biological damage | Rem = Rad x Quality Factor
1 Sv = 100 rem |
Coulombs/kilogram, the Gray, and the Sievert are the SI units for these quantities.
For more detailed information about the meaning of these quantities and units.
Radioactive materials decay at exponential rates unique to each radioisotope. Half-life is the time required for a given amount of some radioactive material to be reduced to one-half of its original activity.
The half-life values for radioisotopes vary widely. For example, the following table shows half-lives for radioisotopes commonly used at the University of Utah:
| Radioisotope |
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| Hydrogen-3 |
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| Carbon-14 |
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| Phosphorus-32 |
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| Phosphorus-33 |
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| Sulfur-35 |
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| Iodine-125 |
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This is the end of the Radiation Basics Module, which is the first of six training modules. The next module is the Background Radiation & Other Sources of Exposure Module.
Go to the Second Module (Background Radiation)
Back to Training Introduction Page